Hybrid Fiber Coax (HFC) is a term that refers to a broadband network based on a combination of optical fiber and coaxial cable. A HFC architecture is often used by cable TV (CATV) operators. In modern cable TV networks, optical fiber is used to transport data between the headend and an optical node deployed in a neighborhood or area of service, while coaxial cable is used to transport data between the optical node and the local houses and locations in the area of service of that optical node.
Over the years, the CATV hybrid Fiber Coax (HFC) architecture has evolved such that the optical node is deployed increasingly closer to the cable customers' premises. Older HFC systems deployed long chains of amplifiers (potentially arranged in a tree structure) between the optical node and cable customers' homes. Over time, amplifier chains have become shorter by segmenting a single long chain of amplifiers to result in multiple smaller chains connected to multiple optical nodes, such that the same area of service previously supported by a single optical node is now serviced by a plurality of optical nodes.
The size of the service domains of an optical node are often quantified not by the length and reach of the coaxial cables, but rather by the number of amplifiers used in the path of the coaxial cable between the optical node and the cable customers. For example an N+6 deployment signifies that there are as many as 6 amplifiers cascaded between the optical node (N) and the customer.
In an amplifier chain, each amplifier typically receives as input a low level RF signal from the end of a coaxial cable segment, amplifies the received RF signal, and outputs the amplified RF signal onto the next coaxial cable segment. Each such amplifier introduces undesirable noise and distortion to the RF signal. For this reason, shorter amplifier chains generally result in a better signal quality delivered to the cable customer. To enable a longer reach between amplifiers, the power level at the output of each amplifier is increased.
Recently, Fiber Deep (FD) deployments have become popular, as they represent the next stage in evolution of cable networks. In a FD deployment, optical fiber is extended from the cable operator's headend or hub deep into the CATV outside plant, close to the customers' premises, into an optical node that produces RF signals for final distribution over the coaxial network. Another term used to describe this architecture is N+0, where N stands for the optical node and zero signifies that there are no (zero) standalone RF amplifiers between the optical node and the customers' premises. Since there are no additional amplifiers that can degrade the RF signal fidelity, such deployments often use optical nodes with very high output integrated power amplifiers. These High Output (HO) optical nodes are designed to drive the RF signal into a distribution coax network directly (i.e., without being amplified by further amplifiers) into cable subscriber homes. A typical high output FD optical node comprises 4 output ports, each of which is driven by a dedicated high output RF power amplifier.
Due to their strict linearity requirements and large active RF bandwidth, power amplifiers in both Remote PHY optical nodes and Remote MACPHY optical nodes normally operate as Class A amplifiers, rendering them very inefficient. A Class A power amplifier is an amplifier in which the amplifier bias is set such that the amplifier is not driven into its cut-off or saturation regions by the signal it amplifies. Thus, a Class A amplifier exhibits the lowest signal distortion levels and has the highest linearity over the other amplifier classes. This is at the expense of lower power conversion efficiency, which is measured as signal output power divided by the total power consumed by the amplifier.
Although advances in amplifiers have been made over the years, with newer GaN (Gallium Nitride) amplifiers having improved efficiency over prior GaAs (Gallium Arsenide) amplifiers, the typical achievable efficiency of the best high output CATV power amplifiers remains lower than 3.5% at the power amplifier output and less than 2% at the optical node output. The most advanced and highest output power amplifiers presently available consume about 18 W of power to enable about 0.3 W RMS RF power at the output of the optical node. In a typical Remote PHY node having 4 node ports, more than half of the power consumed by the Remote PHY node is consumed by the 4 high output RF power amplifiers for those 4 node ports.
FIG. 1 is an illustration of power amplifier 10 which may be used in the prior art. Many power amplifiers in the prior art allow for lowering the amplifier bias current of power amplifier 10 using adjustable bias current control 12 having a fixed predetermined bias that is established at time of manufacture. Also in the prior art, voltage source 14 may use a lower voltage than is typically used with power amplifier 10. In either case, not only will the power consumed by power amplifier 10 be reduced, but the maximum RF signal power of RF out signal 16 that power amplifier 10 can output without generating significant distortions will also be reduced.
For this reason, when power amplifiers are used in optical nodes which require a lower RF signal power than the maximum capability of the power amplifier, it is standard procedure to design and manufacture the optical node to employ a lower current bias in the power amplifier. The level of the current bias for the power amplifier is adjusted down by design to a level which is just sufficient to support the desired RF signal output power without generating distortions. Doing so enables the power consumption of the power amplifier, and the optical node in which it is installed, to be lowered while still allowing for the output signal to be generated with sufficient fidelity. When such bias adjustment is practiced, it is done during the design and/or manufacturing of the optical node. Consequently, the power consumption of the power amplifier, and thus by extension the maximum power of the RF signal output that the power amplifier can support as part of an optical node, is fixed for the life of the optical node.